The invention relates generally to freeze crystallization concentration systems, which are used to separate water from a solution in which solids are dissolved or suspended. More particularly, the invention relates to improved freeze crystallization apparatus and methods utilizing a single pass or one-step crystallization process that is more efficient and economical than existing systems.
The use of freeze-crystallization concentration systems to separate liquid feed streams into a more purified liquid and a concentrate is known. These systems have many uses, including conversion of a contaminated waste water stream into fresh water and concentrate, desalination of sea water, concentration of solutions or suspensions containing food such as orange juice or coffee, and separation from solution of chemicals having different freezing points. In some of these cases, the desired product is purified water, which can be obtained by melting the ice formed in such systems, but in other cases the desired product is the concentrate.
Freeze-crystallization concentration processes operate by taking advantage of the scientific principle that ice crystals, as they freeze, exclude dissolved impurities, including organics, inorganics and volatiles. Thus, the resulting ice crystals consist of purified water.
Generally, freeze-crystallization systems include at least one freeze-crystallizer to form ice crystals and a concentrate, and at least one wash column to separate ice from concentrate and to wash the surface of the ice crystals. Systems which include only one freeze-crystallizer and one wash column are referred to as single stage or one-step systems.
Systems have been employed using two freeze-crystallizers and two wash columns, which are referred to as two-stage systems. One example of a two-stage freeze-crystallization system is described in U.S. Pat. No. 3,885,399 to Robert J. Campbell. The system described in this patent recycles concentrate from the first-stage wash column and uses the recycled concentrate as wash water in the second-stage wash column. Another two-stage freeze crystallization system is described in U.S. Pat. No. 4,091,635 to Abraham Ogman. This patent employs two separate crystallizers, both of which cool the incoming feed stream by direct injection of immiscible secondary refrigerants into the feed. The pressure in the crystallizer is maintained such that the refrigerant can vaporize, withdrawing sufficient heat from the input feed to cause ice crystal formation.
One of the major problems in prior freeze concentration systems has been removing the ice crystals from the concentrate. This problem intensifies with increasing concentration and viscosity. Many methods have been tried or proposed to overcome this problem, but have met with dubious success. For instance, in most cases, use of a one-step system in which the feed stream passes through the freeze-crystallizer only once is insufficient to produce the desired concentration. Thus, it has been proposed to use a second freeze-crystallizer in a two-stage system or to send some of the concentrate back to the first freeze-crystallizer without any ice crystals in the feed stream.
One of the major problems with the two-stage solution lies in duplication of expensive equipment required in two-stages systems, such as wash columns, filters or centrifuges, and refrigeration systems, which results in heat loss and increases the energy cost per pound of water removed. Another problem with two-stage systems is their limited ability to treat highly contaminated feed streams. One reason for this is that when the concentration of the feed stream is relatively high, the ice crystals produced are relatively small and therefore difficult to wash. (The rate of ice crystal growth is inversely proportional to the concentration of the surrounding liquid.) Solutions to this problem, such as diluting the feed, increasing retention time in wash columns, building larger wash columns, providing finer filters and better screens in centrifuges, also result in excessive energy consumption or increased capital costs.
Another significant problem in freeze crystallization systems of the either one- or two-stage variety has been the costly and inefficient design of the freeze-crystallizer apparatus itself. Indirect freeze crystallizers in which the coolant is maintained separate from the feed have been built and patented in many forms. Most common have been the scraped-surface heat exchangers or falling-film heat exchangers, which typically use either rotating motors or hydraulic pressure of the liquid as the driving force. All of these designs have severe limitations including a limitation of the amount of ice crystals produced in a single pass, which at its best has been an ice fraction of about 40%. Rotating scraper crystallizers must be very large by design and hydraulically-driven crystallizers, including the falling film variety, are constrained to particular spatial orientations. The falling film type must operate in a vertical position with the feed stream entering at the top; it is also sensitive to the viscosity of the liquid it processes. Hydraulically-driven shell and tube crystallizers must be operated in a horizontal position. Shell and tube freeze-crystallizers that cool the feed stream by direct injection of the refrigerant into the feed, such as disclosed in the above-mentioned U.S. Pat. No. 4,091,635, suffer from several additional drawbacks. The use of the direct refrigerant, which creates increased pressure in the shell that fluctuates depending on whether the device is being operated, requires a thick tube wall design, with the attendant low heat transfer efficiency and low density of tubes per area, resulting in a large overall size.
As a result of the foregoing problems, there are very few freeze concentration systems that can compete with other separation processes known in the industry, such as distillation, electro-dialysis or reverse osmosis, which are commonly known but expensive methods for removing water from a solution containing dissolved solids like salts and other minerals. Many projects in which freeze concentration systems could have been used successfully have failed because of the above-noted problems, which have restricted the use of such freezing processes and prevented their commercial application.